Advanced, high energy density batteries are required for use in space, military, communication and automotive applications. The power requirements for these application vary from a few tens of watts to a few kilowatts. Most of these applications are mass, volume and cost sensitive. Certain jurisdictions such as California have mandated that an increasing percentage of automobiles must be powered by electricity within the next few years. The lead-acid battery, though reliable and capable of many recharge cycles, is too heavy and has too low an energy to weight ratio. State of the art Ag-Zn and Ni-Cd batteries have poor charge retention properties and are also too heavy and bulky for use on space missions and in some cases do not meet the life and environmental requirements. They also have poor charge retention properties.
Ambient temperature, secondary lithium batteries have several intrinsic and potential advantages including higher energy density, longer active shelf life, and lower self discharge over conventional Ni-Cd, Pb-acid and Ag-Zn batteries. Successful development of these batteries will yield large pay-offs such as 2--3 fold increase in energy storage capability and a longer active shelf life of 2 to 4 years over Ni-Cd. These cells are most suitable for small spacecraft application requiring less than 1 kW power. Secondary lithium batteries are presently being considered for a number of advanced planetary applications such as: planetary rovers (Mars Rover, Lunar Rover), planetary space craft/probes (MESUR, AIM, ACME Mercury Orbiter) and penetrators. These batteries may also be attractive for astronaut equipment, and Geo-SYN spacecraft.
Secondary lithium cells under development employ lithium metal as the anode, transition metal chalcogenides such as oxides and sulfides, for example, TiS.sub.2, MoS.sub.2, MoS.sub.3, NbSe.sub.3, V.sub.2 O.sub.5, Li.sub.x Mn.sub.2 O.sub.4, Li.sub.x CoO.sub.2, LiV.sub.3 O.sub.8 and Li.sub.x NiO.sub.2 as the cathode and liquid organic or solid polymeric electrolytes. During discharge of the cell, lithium metal is oxidized into lithium ions at the anode and lithium ions undergo an intercalation reaction at the cathode. During charge reverse processes occur at each electrode.
Rechargeable batteries using lithium anodes and transition metal oxide or chalcogenide cathodes were extensively investigated as candidates for powering electric vehicles about ten years ago. These batteries have not proved to be acceptable for this use due to poor cycle-life performance and concerns about their safety. Lithium is a very reactive material. When freshly deposited, lithium is highly active and can react with most inorganic and organic electrolytes which results in lower cycling efficiency. Prolonged cycling of secondary lithium cells produces large quantities of finely divided, dendritic lithium increasing the risk of thermal runaway. Hence, ambient temperature secondary lithium cells are potentially unsafe after (1) extended cycling, or after (2) being subjected to overcharge followed by over discharge.
The limited cycle life of state of the art, ambient temperature secondary lithium cells is believed due to formation of shorts during cycling. The best state of the art cells are found to provide 200-300 cycles at 100% DOD. However, 20-30% of these cells failed even before 100 cycles due to the formation of shorts. Some cells even vented after the formation of shorts.
Dendritic lithium growth and degradation of electrolyte by reaction with pure lithium can be reduced by use of mixed solvent electrolytes or lithium anodes that undergo displacement or insertion reactions at activities less than unity. Several lithium alloys and intercalation compounds are under investigation as candidates for Li anode materials. The best performing of these alternative anode materials are LiC.sub.6 or LiAl. These alternative electrodes do improve reversibility and cycle life of the cells. However, their use results in a reduction of cell specific energy and power density. The energy and power reduction might be an acceptable trade-off if there was a significant improvement in cell cycle life, performance and safety. These two alternate anode materials have other limitations. LiC.sub.6 demonstrates poor ability to retain charge and LiAl has poor mechanical strength during cycling. Other anode materials that undergo insertion reactions such as graphite, LiAlX ternary alloys and other intercalation compounds are being investigated for use in an ambient temperature, secondary lithium cell. These approaches do delay the formation of shorts and extend the cycle life of the cells.
Presently microporous polypropylene and non-woven glass paper are the materials of choice for use as separators in secondary lithium cells. These materials do not prevent lithium dendrites from shorting the cell. The longest lithium dendrite extending from the anode will penetrate the separator and will eventually contact the cathode and short the cell. For safety reasons a thermal fuse has been provided in these cells. The surface of the separator is coated with a wax coating that melts when the temperature of the separator rises as the cell is shorted. The wax coating melts and blocks all the pores in the separator.
Referring now to FIG. 1, a prior art cell 10, comprises an electrolyte impervious housing 12 containing a lithium anode 14, a set of porous separators 16, 17 such as Celgard, a porous polypropylene, containing liquid electrolyte and a composite chalcogenide cathode 18 usually supported on a current collector 20 such as nickel or stainless steel mesh. The cell contains a thermal fuse layer 22 such as a low melting polyethylene wax which is inert in the electrochemical environment of the cell between the two inert separators 16, 17.
During cycling of the cell 10, a lithium dendrite 30 will form and extend normal from the surface 32 of the anode 14. Referring now to FIG. 2, as it grows the dendrite 31 will at first be blocked by the first separator 16. As cycling of the cell 10 continues the dendrite 33 will penetrate the first separator 16, thermal fuse layer 22 and be temporarily stopped by the second separator 17. However, eventually one of the dendrites 35 will penetrate the second separator 17 and will grow and contact the surface 24 of the cathode 18 and will cause the cell to short. The temperature within the cell will be raised to the melting temperature of the thermal fuse layer 22. The thermal fuse layer 22 will melt and will block all the pores in both separators 16, 17. The cell reaches open circuit condition and becomes non-functional.